The present invention relates to a gas flow meter for automotive control and more particularly to a noise reduction circuit, to an adjustment circuit, to a reduction in the number of adjustment terminals and output terminals, and to an output circuit.
A gas flow meter for detecting an air flow in internal combustion engines has been in use. An example of the gas flow meter is a constant temperature control hot wire type gas flow meter described in the Journal of Fluid Mechanics, vol. 47 (1971), pp577-599. FIG. 25 shows an outline configuration of a gas flow detection circuit DECT1 applying the constant temperature control heat wire type gas flow meter. This gas flow detection circuit mainly comprises an operational amplifier OP1, a power transistor Tr1, a heating resistor (also called a hot wire) Rh, a gas temperature measuring resistor (also called a cold wire) Rc and resistors R1, R2 and keeps the temperature of the heating resistor Rh constant at all times, i.e., keeps its resistance constant by maintaining a bridge balance using the operational amplifier OP1. As the gas flow increases, heat taken from the heating resistor Rh increases resulting in an increased heating current. Because this heating current is proportional to a voltage between terminals of the resistor R1, the measurement of this voltage can determine the gas flow. The voltage output produced by the current detection resistor R1 is processed by an adjust circuit having a predetermined input/output characteristic so that the voltage output provides a predetermined signal characteristic required of the gas flow meter.
There is another gas flow detection circuit DECT2, as shown in FIG. 26, in which heat sensing resistors Ru, Rd for measuring gas flow temperatures are arranged upstream and downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh. The resistor Ru on the upstream side is cooled by the gas flow to lower its resistance and the resistor Rd on the downstream side receives a gas flow heated by the heating resistor Rh to raise its temperature and therefore its resistance. This changes the potential at a connecting point between Ru and Rd and thus measuring this voltage can determine the gas flow.
Still another gas flow detection circuit DECT3 as shown in FIG. 27 is available, in which a total of four heat sensing resistors for measuring gas flow temperatures are arranged two upstream and two downstream of the heating resistor Rh of the constant temperature control hot wire type gas flow meter so that they are influenced by heat from the heating resistor Rh, and in which one pair of resistors Ru1, Rd1 are serially connected in an upstream-downstream order and another pair of resistors Rd2, Ru2 are serially connected in a downstream-upstream order to form a bridge and measure a potential difference between two connecting points. The resistors Ru1, Ru2 on the upstream side are cooled by the gas flow to lower their resistances and the resistors Rd1, Rd2 on the downstream side receive a gas flow heated by the heating resistor Rh, raising their temperatures and therefore their resistances. This changes the potential difference in the bridge and thus measuring this voltage difference can determine the gas flow.
The electronic circuits that adjust the output characteristic of a gas flow meter mounted on motor vehicles are subject to various surges and overvoltages, as specified in the International Standard Organization (ISO) 7637-1, 7637-3 standard and Japan automotive standard (JASO) D001-94. These standards are intended to prevent undesired operations or failures of electronic circuits due to surge voltages caused by ignition of engine, overvoltages caused by batteries stacked in two tiers at time of starting engine in cold environment, and high frequency noise caused by other electronic devices. On the other hand, the electronic circuits are constructed in the form of IC circuits for reducing the manufacturing cost and, in recent years, to meet the emission control requirements the gas flow meter is increasingly required to raise its precision in line with the sophistication of engine control functions. Further, because the service temperature range is as wide as xe2x88x9240xc2x0 C. to 130xc2x0 C., measures should be taken to prevent a possible change in output due to temperature variations.
For surges and overvoltages, a variety of overvoltage protection circuits have been in use. One such example is a protection circuit using a Zener diode ZD and a current limiting resistor R as shown in FIG. 28.
The circuit of FIG. 28 is one type of a commonly used constant voltage circuit in which a voltage applied to a connection terminal VBB for the battery causes a current to flow through the current limiting resistor R to the Zener diode ZD. When an overvoltage is applied, the voltage of the power supply terminal Vcc to various circuits is clamped by a Zener voltage of the Zener diode ZD to put an overvoltage protection into acton.
Further, JP-A-9-307361 proposes as a conventional technology an overvoltage protection circuit that uses an overvoltage detection circuit made up of a resistor and a Zener diode and a switching circuit made up of bipolar transistors.
The overvoltage protection circuit described in this official gazette is intended for protecting microwave FETs (field-effect transistors). When an overvoltage higher than a voltage sum of the Zener voltage of the Zener diode and the base-emitter voltage of the switching transistor is applied to the power supply terminal, the switching circuit is operated to cut off the load from the power supply line and thereby prevent the overvoltage from being impressed on the load.
The voltage outputs of the flow detection circuits DECT1-3 in FIG. 25 to FIG. 27 need adjustments in zero point and span (output range) to produce the required sensor output characteristics. This adjust circuit is mainly an analog circuit at present but a higher precision adjustment is considered possible by using a digital circuit.
Table 1 shows comparison between an analog circuit and a digital circuit (xe2x80x9cCMOS Analog Circuit Design Techniquexe2x80x9d published by Triceps (1998), compiled under the supervision of Iwata).
The analog circuit has a small size and a small power consumption compared with the digital circuit. But the use of such devices as resistors causes manufacturing variations and other variations due to aged deterioration, and thus the analog circuit has less precision and stability than the digital circuit. The digital circuit, while it is superior to the analog circuit in terms of precision and stability, has a larger circuit size and a larger power consumption. The rapid advance in the integrated circuit manufacturing technology in recent years, however, has enabled micro-fabrication and therefore reduced the circuit size and power consumption. The digital circuit is now finding many applications in various industrial fields. Example applications of a digital adjust circuit to the gas flow meter are found in Japanese Patent No. 3073089 and JP-A-8-62010 and JP-A-11-118552.
FIG. 29 shows comparison between an analog adjustment and a digital adjustment in the adjust circuit of the gas flow meter.
An outline circuit configuration for analog adjustment shown in FIG. 29 comprises an operational amplifier OP2, trimming resistors Rs1, Rz1 and resistors Rs2, Rz2. This circuit trims the trimming resistors Rz1, Rs1 to adjust the voltage output from the flow detection circuit DECT and thereby adjust the zero point and span to produce an output for a desired gas flow. As the trimming resistors Rs, Rz, thin-film resistors printed on a hybrid IC or thin-film resistors on IC may be used. In trimming the resistors, a laser trimmer may be used. The laser trimmer has a disadvantage that trimming with high precision takes time and re-trimming cannot be done. Further, because only a two-point adjustment is made, it is difficult for the laser trimmer to make a complicated adjustment on the output characteristic, such as a non-linear adjustment. In the analog circuit, when the output specification for the gas flow is changed, the resistance value needs to be redesigned and, in some cases, it is necessary to redesign the hybrid IC substrate pattern, which in turn increases the man-hour of designing works.
In the case of the digital adjust circuit of FIG. 29, since the output specification can be changed by simply changing an adjust coefficient while leaving the circuit pattern intact, the number of design steps can be reduced. As an example digital adjust circuit, a method described in Japanese Patent No. 3073089 has been proposed. A rough circuit configuration for the digital adjustment is as follows. The voltage output from the flow detection circuit DECT is converted into a digital value by an analog-digital converter AD. Based on the digital value, a digital processor CALC calculates the zero point and span adjustments, which are then converted by a digital-analog converter DA into an analog signal which is an analog output for a desired gas flow. The adjust coefficient used in this calculation is stored in a storage device MEM such as PROM. Further, the digital processor CALC, because of its ability to easily perform non-linear calculations, can make non-linear adjustments as well as zero point and span adjustments during the output adjustment. With this non-linear adjustment, the adjustment accuracy is within xc2x12%.
Another example configuration for the digital adjustment is found in JP-A-11-118552. While its configuration is similar to that of the digital adjust circuit of FIG. 29, this circuit reduces its circuit size by using an oversampling type analog-digital converter including a delta-sigma modulator as an analog-digital converter AD.
Still another example configuration for the digital adjustment is found in JP-A-2000-338193. The adjust coefficient used by the processor in executing the adjustment calculation is written into a storage device such as PROM through a terminal of a digital input/output circuit that communicates with external circuits of the sensor. This official gazette describes that a third-degree polynomial is used for the adjustment calculation.
A further example configuration for the digital adjustment is found in JP-A-11-94620. This circuit converts a flow signal from the gas flow detection circuit into a rectangular wave signal and counts up a counter at a certain rate only while the rectangular wave is xe2x80x9c1xe2x80x9d. To this count value is added the adjust coefficient to produce an output.
Because the heating current flowing through the heating resistor Rh is not affected by voltage variations in the power supply (for example, battery), the voltage output of the gas flow detection circuit DECT1 has a non-ratiometric characteristic. As output specifications of the gas flow meter, there are ratiometric analog and digital output specifications in addition to the non-ratiometric analog output specification. A circuit configuration that realizes the ratiometric analog output circuit is described in JP-A-2-85724. This circuit divides an external ratiometric output reference voltage into smaller voltages by two resistors and inputs the divided voltages to an operational amplifier to realize a ratiometric output. With a sum of the two resistors set to about 10 kilo-ohm, the current to be supplied from the reference voltage is relatively small at about 0.5 mA. An example of the digital output circuit is disclosed in JP-A-8-247815. This circuit configuration comprises at least a constant temperature control circuit, a zero point/span adjust circuit and a voltage control oscillator, all integrated into one chip.
Another configuration is described in JP-A-5-203475 in which an analog output and a digital output are produced by a single circuit board. In this configuration, a single circuit board is provided with both an analog output terminal and a digital output terminal, and both analog and digital outputs are supplied to an output connector which selects and uses one of the two output signals or only one of the outputs is connected through wire to the output connector.
When the gas flow meter circuit is integrated into a digital circuit to reduce the cost and size of the gas flow meter and enhance the accuracy of the output by adjustment, the conventional technique described above is not optimized and thus has some problems that cannot be solved by conventional technology.
C-MOSs are used for enhancing the level of circuit integration and for building the gas flow meter circuit with a digital circuit. The C-MOSs, however, are easily affected by surges and overvoltages compared with bipolar transistors used in analog circuits and thus need countermeasures.
In an overvoltage protection circuit shown in FIG. 28, when a current through a connected circuit is large, a resistance of a current limiting resistor must be reduced to prevent a voltage drop. In this case, the electric withstandability of a Zener diode ZD is increased large enough so that it can withstand an overcurrent. This results in an increase in size and cost of components, which is not desirable.
A circuit described in JP-A-2000-338193 performs a nonlinear adjustment using a cubic equation. When the nonlinear adjustment needs to be done with a quatic equation or higher order function, the calculation time naturally increases. In addition, if individual output characteristics have steep characteristic changes with respect to an ideal characteristic, the output characteristics may often not be able to be adjusted with such a polynomial.
Next, in integrating the electronic circuit of the gas flow meter into a digital circuit, because adjustment coefficients need to be written into a programmable storage device during the adjustment process, terminals must be added. Further, there are different specifications on the sensor output, i.e., a ratiometric analog output, a non-ratiometric analog output and a digital output. For reduced manufacturing cost, it is necessary during the integration process to make provisions for coping with all these specifications. Simply adding terminals to meet this requirement, as described in Japanese JP-A-11-94620, results in an increase in the chip area, which should be avoided.
Next, when the adjustment calculation is digitized, a digital-analog converter may be required at the output stage. The digital-analog converter includes an amplifier circuit for a signal output to external circuits and thus its current consumption reaches several mA. When the digital-analog converter is to be driven by using an external reference voltage to produce a ratiometric output, if the maximum current supplied from its power supply is small, the digital-analog converter cannot be operated. This raises a problem that the reference voltage cannot be connected directly to the power supply terminal of the digital-analog converter.
It is therefore an object of the present invention to provide a means which solves various problems encountered when reducing the cost and size of the gas flow meter, enhancing the integration-level of electronic circuits, making the output characteristics more accurate and adjustable, and transforming the circuits into digital circuits.
To achieve the above objective, the present invention discloses the following configuration:
(1) A gas flow meter comprising:
a gas flow detection circuit for detecting a current flowing through a resistor installed in a gas passage and a voltage generated across the resistor and outputting a voltage signal representing a gas flow passing through the gas passage;
a noise reduction circuit for reducing external noise; and
a digital adjust circuit for digitally adjusting a signal representing the detected gas flow and outputting the adjusted signal;
wherein a voltage signal based on the signal adjusted by the digital adjust circuit is output.
(2) Preferably, the gas flow meter according to item (1), wherein the digital adjust circuit includes:
a digital conversion circuit for converting an output from the gas flow detection circuit into a digital signal;
an adjust means for adjusting the digital signal to produce a desired output characteristic; and
a regulator circuit for supplying a reference voltage to the digital conversion circuit and/or the adjust means.
With the above arrangement, the digital adjustment type gas flow meter has a more appropriate circuit configuration.
(3) Preferably, a gas flow meter comprising: a gas flow detection circuit for detecting a gas flow passing through a gas passage;
an adjust circuit for adjusting an output characteristic to a desired output characteristic and outputting it; and
a noise reduction circuit including an overvoltage protection circuit and supplying to the gas flow detection circuit and the adjust circuit a voltage whose surges and overvoltages applied to a power supply terminal are reduced;
wherein there are two or more voltage supply paths for supplying different voltages to the gas flow detection circuit and the adjust circuit through the overvoltage protection circuit.
With this arrangement, the minimum required voltage can be properly supplied to the gas flow detection circuit and the adjust circuit of the gas flow meter.
(4) Preferably, the gas flow meter according to item (3), wherein in one of the voltage supply paths for supplying a voltage having reduced surges and overvoltages to various circuits, a voltage limiter circuit that turns on when applied with a voltage in excess of a predetermined voltage is connected between a voltage supply terminals and a ground terminal and a current limiting resistor is connected between the power supply terminal and the voltage supply terminals; in the other voltage supply path, another current limiting resistor is connected between the power supply terminal and the voltage supply terminals; and an overvoltage protection circuit is provided in which a diode is connected between each of the voltage supply terminals.
With this arrangement, the overvoltage protection circuit has a more appropriate configuration.
(5) Preferably, the gas flow meter according to item (3), wherein in all of the voltage supply paths for supplying a voltage having reduced surges and overvoltages to various circuits, a voltage limiter circuit that turns on when applied with a voltage in excess of a predetermined voltage is connected between voltage supply terminals and a ground terminal and a current limiting resistor is connected between the power supply terminal and the voltage supply terminals; and
an overvoltage protection circuit is provided in which the current limiting resistors connected to the respective voltage supply terminals have different resistances.
With this arrangement, the noise reduction circuit has a more appropriate configuration.
(6) Preferably, the gas flow meter according to item (4) or (5), further including an overvoltage protection circuit having an additional diode connected between the voltage supply terminals and the ground terminal.
With this arrangement, the overvoltage protection circuit has a more appropriate configuration.
(7) Preferably, the gas flow meter according to any one of items (3) through (6), wherein a part or all of devices included in the overvoltage protection circuit, the gas flow detection circuit and the adjust circuit are formed in the same integrated circuit.
With this arrangement, the circuit can be reduced in size.
(8) Preferably, the gas flow meter according to any one of items (3) through (7), wherein the number of the voltage supply paths are two; and
a circuit connected to a higher supply voltage is an operational amplifier in the gas flow detection circuit and a circuit connected to a lower supply voltage is a regulator that supplies a voltage to the digital adjust circuit.
With this arrangement, the digital adjustment type gas flow meter has a more appropriate configuration.
(9) Preferably, a gas flow meter preferably comprising:
a gas flow detection circuit for outputting a voltage signal representing a gas flow passing through a gas passage; and
an adjust circuit for adjusting the voltage output from the gas flow detection circuit;
wherein an input range of the voltage signal entered into the adjust circuit is divided in two or more and, in each divided range, a different adjustment calculation formula is determined in advance;
wherein a means is provided which selects the adjustment calculation formula according to an input value of the voltage signal entered into the adjust circuit and performs adjustment calculation to produce an output value.
With this arrangement, the gas flow meter can perform a more precise adjustment during the output characteristic adjustment.
(10) Preferably, the gas flow meter according to item (9), wherein the adjust circuit is a digital adjust circuit which digitally adjusts the signal representing the detected gas flow and outputs the adjusted signal.
With this arrangement, the adjustment as described in item (9) can be realized.
(11) Preferably, the gas flow meter according to item (9) or (10), wherein the adjust circuit has input/output characteristics represented by each of the adjustment calculation formulas expressed as a first-degree function of y=axc2x7x+b where x is an output value of the gas flow detection circuit, i.e., input value for the adjustment calculation, y is an output of the adjustment calculation, and a and b are adjustment coefficients.
With this arrangement, the calculation time can be shortened.
(12) Preferably, the gas flow meter according to any one of items (9) through (11), further including:
a temperature sensor; and
a digital conversion circuit for converting an output of the temperature sensor into a digital value;
wherein the adjust circuit also uses the output of the temperature sensor in performing the adjustment calculation.
With this arrangement, the temperature adjustment can be made.
(13) Preferably, the gas flow meter according to item (12), wherein the adjust circuit has an input/output characteristic expressed by
y=(a1xc2x7t+a2)xc2x7x+(b1xc2x7t+b2)
where x is an output value of the gas flow detection circuit, t is an output value of the temperature sensor, and a1, a2, b1 and b2 are adjustment coefficients.
With this arrangement, the digital adjust circuit can perform an appropriate adjustment. (14) Preferably, the gas flow meter according to item (11) or (13), wherein the adjust circuit writes the adjustment coefficients a, a1, a2, b, b1 and b2 into a programmable storage device.
With this arrangement, the digital adjust circuit has an appropriate circuit configuration.
(15) Preferably, the gas flow meter according to item (11) or (13), wherein the adjust circuit writes the adjustment coefficients a, a1, a2, b, b1 and b2 into an erasable and programmable storage device.
With this arrangement, the digital adjust circuit has an appropriate circuit configuration.
(16) Preferably, a gas flow meter comprising:
a gas flow detection circuit for outputting a voltage signal representing a gas flow passing through a gas passage;
an adjust circuit for adjusting the voltage output of the gas flow detection circuit;
a storage device for storing data for adjustment; and
a data input/output circuit;
wherein the data input/output circuit has two external data communication terminals for writing adjust data from outside into the storage device and for reading data from the storage device to the outside.
With this arrangement, the gas flow meter can be made small in size.
(17) Preferably, the gas flow meter according to item (16), wherein the adjust circuit has a means which, after a predetermined number, two or more, of pulses have been supplied to one of the external data communication terminals of the data input/output circuit, allows the adjust circuit to enter into a data communication mode where it transfers data between the storage device and external circuits. With this arrangement, the adjust circuit can be prevented from undesirably entering into the data communication mode even when pulse noise is impressed during normal operation.
(18) Preferably, a gas flow meter comprising:
a gas flow detection circuit for outputting a voltage signal representing a gas flow passing through a gas passage;
an adjust circuit for adjusting the voltage output of the gas flow detection circuit; and
a storage device for storing data for adjustment;
wherein the adjust circuit retrieves as the output signal of the detected gas flow a ratiometric analog output, a non-ratiometric analog output and a digital output and selects one of these output signals by an output selection means provided in the adjust circuit.
With this arrangement, a single gas flow meter can cope with a variety of output specifications and contribute to standardization and reduction in manufacturing cost.
(19) Preferably, the gas flow meter according to item (18), wherein circuits for producing the ratiometric analog output, the non-ratiometric analog output and the digital output are formed on the same integrated circuit.
With this arrangement, the circuit can be reduced in size.
(20) Preferably, the gas flow meter according to item (16) or (18), wherein the external data communication terminals serve as a detected flow output terminal.
With this arrangement, the gas flow meter can be reduced in size.
(21) Preferably, a gas flow meter comprising:
a gas flow detection circuit for detecting a current flowing through a resistor installed in a gas passage and a generated voltage and outputting a voltage signal representing a gas flow passing through the gas passage;
a digital conversion circuit for converting the detected gas flow into a digital signal; and
a digital adjust circuit for digitally adjusting the digital signal and outputting the adjusted digital signal;
wherein a voltage signal based on the digital signal adjusted by the digital adjust circuit is output, and
the digital conversion circuit has a means for selecting either a single-phase input or a differential input.
With this arrangement, the adjust circuit can deal with either a gas flow detection circuit with a single-phase output or a gas flow detection circuit with a differential output
(22) Preferably, a gas flow meter comprising:
a gas flow detection circuit for detecting a current flowing through a resistor installed in a gas passage and a voltage generated across the resistor and outputting a voltage signal representing a gas flow passing through the gas passage;
a digital conversion circuit for converting the detected gas flow into a digital signal;
a digital adjust circuit for digitally adjusting the digital signal and outputting the adjusted digital signal; and
an analog conversion circuit for receiving the adjusted digital signal and converting it into an analog signal;
wherein the analog conversion circuit is driven by a voltage based on an external reference voltage and a voltage follower circuit is arranged between a reference voltage terminal and a power supply terminal which drives the analog conversion circuit.
With this arrangement, the digital-analog converter can be operated even when the current supplied from the external reference voltage is small.